1. Technical Field
The present invention relates generally to an improvement on an optical information recording/reproducing system designed to record and reproduce data on and from an optical information storage medium in which data can be written once, such an optical information storage medium, and a manufacturing method thereof.
2. Background of Related Art
FIG. 1 shows a conventional write-once optical disc, as disclosed in Japanese Patent First Publication No. 2-185737 assigned to the same assignee as that of the present application, which has compatibility with a CD (compact disc) in that it has the same standards for reflectivity, modulation of high-frequency signals, symmetry of high-frequency signals, output of tracking signals, and crosstalk.
The write-once optical disc, as indicated at numeral 100, includes generally a transparent substrate 1, an organic film 2, and a metallic film 3. The transparent substrate 1 has formed therein a tracking guide groove G that is a blank groove. The organic film 2 is formed on the transparent substrate 1 and, when irradiated with a recording laser beam L having a preselected wavelength, absorbs a given portion thereof to decrease its refractive index. The metallic film 3 is formed on the organic film 2 for optical reflection.
The organic film 2 is formed so as to reflect 70% or more of the laser beams L before data is recorded thereon in order to show the same reflectivity as that of the CD. The decrease in reflectivity when the organic film 2 is irradiated with the laser beam L after data is recorded thereon is achieved by decreasing the thickness of the organic film 2 to bring the equivalent optical length of the laser beam L close to .tau./4 (.tau.: wavelength).
In order to meet requirements for compatibility with the CD, the contrast between pits and lands, or signal-to-noise ratio of a reproduced signal is increased, taking the level of a tracking signal into account. This may be accomplished by decreasing the depth of the tracking guide groove G to increase the reflectivity of the laser beam L before recording or alternatively increasing the depth of the tracking guide groove G to bring the equivalent optical length of the laser beam L close to .tau./2.
FIG. 2 shows another type of write-once optical disc 200, as taught in Japanese Patent First Publication No. 4-205738 assigned to the same assignee as that of this application, which is designed to have compatibility with the CD and has formed thereon a ROM (Read-Only Memory) area retaining information permanently and a RAM (Random-Access Memory) area allowing data to be written therein at all times. In the drawing, the same reference numbers as employed in FIG. 1 refer to the same parts.
The transparent substrate 1 has a diameter of 120 mm. The ROM area occupies an inner portion of the transparent substrate 1 ranging from 25 mm to 35 mm in diameter and has formed therein 130 nm-depth pits Go representing signals meeting the CD-ROM standards. The RAM area occupies an outer portion of the transparent substrate 1 ranging from 35 mm to 58 mm in diameter and has a V-shaped tracking guide groove Ga which is absolute time-modulated using the Wobbling with an amplitude of 30 nm to have a depth of 55 nm and a width of 0.5 .mu.m and formed at a track pitch of 1.8 .mu.m. The organic film 2 is formed on the entire surface of the transparent substrate 1 which, when irradiated with the laser beam L having a wavelength of 780 nm, absorbs a given portion thereof to change the refractive index thereof. The reflective metallic film 3 is formed on the organic film 2. The guard film 4 is formed on the metallic film 3.
The matching of the reflectivity of the ROM area with that of the RAM area is achieved by changing the depths of the pits Go and the guide groove Ga in the ROM area or changing the thickness of the organic film 2.
The above write-once optical discs 100 and 200, as described above, show the same reflectivity as that of an information storage surface of the CD and have the organic films 2 on which additional information can be recorded and which is formed on the transparent films 1 having formed thereon the tracking grooves G and Ga, thereby making it easy to reproduce the additional information recorded on the organic films 2 in a conventional CD player.
The write-once optical disc 200 can read information (e.g., tracking information, address information such as track numbers, or money information) out of the ROM area prior to reading information out of the ROM area, record any valuable information usable as money, for example, on the RAM, and reduce the amount of the valuable information on the RAM as needed, but has the drawback in that there is a possibility of the valuable information on the RAM being altered by unauthorized persons.